The present invention relates to ultrasonic motor drive apparatuses, and particularly to an ultrasonic motor drive apparatus having a switching-type power source.
Ultrasonic motors use ultrasonic vibrations as the driving force. In a progressive wave-type ultrasonic motor, a stator is comprised of an annular elastic body and a piezoelectric body which are attached to each other, and a rotor fixed to a drive shaft is pressured to contact the stator. The piezoelectric body is supplied with drive signals at a fixed frequency and in two phases, a sine wave and a cosine wave that are 90.degree. different in phase. The piezoelectric body produces mechanical vibrations in response to the two-phase drive signals to cause in the elastic body ultrasonic vibrations (progressive waves) that move in the annular direction along the elastic body. The progressive waves rotate the rotor pressure-contacted with the elastic body. The progressive waves rotates the rotor pressure-contacted with the elastic body.
A drive apparatus which generates the drive signals is comprised of a microcomputer, an oscillator circuit, a switching control circuit, a voltage generator circuit, a drive signal generator circuit, a band pass filter and the like. In the drive apparatus, the voltage generator circuit generates a direct current (d.c.) voltage, and the drive signal generator circuit converts it into an alternating current (a.c.) voltage of a fixed oscillation frequency. The drive apparatus applies the a.c. voltage to the piezoelectric body to drive the ultrasonic motor.
Various circuit configurations are proposed for the drive signal generator circuit and the voltage generator circuit. A switching-type power source circuit is generally known. This circuit uses a transformer having a primary coil and a secondary coil. A switching device such as a metal oxide semiconductor field effect transistor (MOSFET) is connected to the primary coil to which the d.c. voltage is supplied. The MOSFET turns on and off the primary coil in response to a switching signal to causes the secondary coil to generate the a.c. voltage which has a boosted voltage level.
In this type of circuit configuration, a surge voltage is likely to develop at the drain side of the MOSFET as shown at time point C in FIG. 7, or oscillation is likely to occur as shown at time point D in FIG. 7 due to the inductance (L) of the transformer and the capacitance (C) of the MOSFET when the MOSFET is turned off.
The ultrasonic motor is used, for instance, in a tilting device and a telescopic device of a vehicle steering system. In this system, the d.c. voltage of a vehicle battery (about 12 V) is converted to the a.c. voltage (about 200 Vrms).
In this instance, both the ultrasonic motor and the drive apparatus are grounded to a vehicle chassis as the other vehicle electrical apparatuses such as a radio receiver are. The drive apparatus tends to generate radiation noises and an electrical wire connecting the drive apparatus and the ultrasonic motor tends to generate wire transmission noises if the surge voltage develops or the L-C oscillation occurs. Thus, the radio receiver produces noise sounds from its speakers.
It is therefore proposed in JP-A-11-191971 to connect a resistor and a capacitor in series between the gate terminal and the drain terminal of the MOSFET so that the surge voltage is suppressed. However, the surge voltage suppression is reduced, if the gate current is increased to speed up the turning on of the MOSFET.
It is also proposed in JP-A-11-191978 to connect ferrite beads between the drain terminal of the MOSFET and the transformer so that the high frequency oscillation is suppressed. However, the surge voltage cannot be suppressed at the time of turning off the MOSFET because the ferrite beads operate as an inductor. Therefore, the MOSFET is required to have a high rated voltage resulting in a high cost. Further, the MOSFET results in a large size, if it is required to have a high rated voltage and an increased drain current.